Engineering Steel with a Bainitic Structure, Forged Parts Produced Therefrom and Method for Producing a Forged Part

ABSTRACT

An engineering steel including (in wt.-%) up to 0.25% C, up to 0.45% Si, 0.20-2.00% Mn, up to 4.00% Cr, 0.6-3.0% Mo, 0.004-0.020% N, up to 0.40% S. 0.001-0.035% Al, 0.0005-0.0025% B, up to 0.015% Nb, up to 0.01% Ti, up to 0.10% V, up to 1.5% Ni and up to 2.0% Cu, remainder iron and unavoidable impurities, wherein the Al content % Al, the Nb content % Nb, the Ti content % Ti, the V content % V and the N content % N meet the following condition: % Al/27+% Nb/45+% Ti/48+% V/25&gt;% N/3.75. The steel has a yield strength of at least 750 MPa, a tensile strength of at least 950 MPa and a structure having at least 80 vol.-% bainite and a total of a maximum of 20 vol.-% of retained austenite, ferrite, perlite and/or martensite.

The invention relates to an engineering steel with high strength and a structure comprising at least 80 vol.-% bainite.

The invention also relates to a forged part, produced from such an engineering steel.

Finally, the invention relates to a method for producing a forged part from an engineering steel according to the invention.

Where, in the following, details of alloys or other steel compositions are provided in “%”, in each case these relate to weight, unless expressly indicated to the contrary.

All mechanical properties indicated in the present text of the steel according to the invention and any steels cited for comparison, unless indicated to the contrary, have been determined according to DIN EN ISO 6892-1.

As Dipl.-Ing. Christoph Keul et al. report in the article “Entwicklung eines hochfesten duktilen bainitischen (HDB) Stahls für hochbeanspruchte Schmiedebauteile” [Development of a high-strength, ductile, bainitic (HDB) steel for highly-stressed forged components], appearing in the Schmiede-Journal, issue of September 2010, published by Industrieverband Massivumformung e.V., particularly in the forging industry there is a demand for steel material concepts which offer the possibility of achieving high strength and ductility and at the same time short process chains for their production. The article also states that in this regard promise has been shown by materials with a bainitic structure, in which good strength and ductility properties are combined without the need for additional heat treatment, characterised by a tensile strength of more than 1,200 MPa, a yield strength of more than 850 MPa and an elongation at rupture of more than 10% with a notch impact energy of 27 J at room temperature. As an example of alloying concepts, offering such properties, the article presents a steel with (in wt.-%) 0.18% C, 1.53% Si, 1.47% Mn 0.007% S, 1.30% Cr, 0.07% Mo, 0.0020% B, 0.027% Nb, 0.026% Ti, 0.0080% N, retained iron and unavoidable impurities and a steel with 0.22% C, 1.47% Si, 1.50% Mn, 0.006% S, 1.31% Cr, 0.09% Mo, 0.0025% B, 0.035% Nb, 0.026% Ti, 0.0108% N, retained iron and unavoidable impurities.

Another development which is similarly aimed at a steel for producing drop-forged parts, which without additional heat treatment possess high strength and at the same time high ductility, is described in EP 1 546 426 B1. The steel known from this patent specification contains (in wt.-%) 0.12-0.45% C, 0.10-1.00% Si, 0.50-1.95% Mn, 0.005-0.060% S, in each case 0.004-0.050% Al and Ti, in each case up to 0.60% Cr, Ni, Co, W, Mo and Cu, up to 0.01% B, up to 0.050% Nb, 0.10-0.40% V, 0.015-0.04% N and retained iron and unavoidable impurities, provided that the product of the V and N contents of the steel is 0.0021-0.0120, that the S-content % S, the Al content % Al, the Nb content % Nb and the Ti content % Ti, meet the condition 1.6×% S+1.5×% Al+2.4×% Nb+1.2×% Ti=0.040-0.080% and the Mn content % Mn, the Cr content % Cr, the Ni content % Ni, the Cu content % C and the Mo content % Mo meet the condition 1.2×% Mn+1.4×% Cr+1.0×% Ni+1.1×% Cu+1.8×% Mo=1.00-3.50%.

It is considered essential here that the necessary improvement in ductility is achieved by reducing the carbon content in the steel. The essential loss of strength that accompanies this according to the prior art is balanced out by the usual alloying elements, the contents of which are coordinated such that strengthening through mixed crystal formation results.

Furthermore, from DE 697 28 076 T2 (EP 0 787 812 B1) a process for producing a forged steel part is known, in which a steel with (in wt.-%) 0.1-0.4% C, 1-1.8% Mn, 0.15-1.7% Si, up to 1% Ni, up to 1.2% Cr, up to 0.3% Mo, up to 0.3% V, up to 0.35% Cu and in each case optionally 0.005-0.06% Al, 0.0005-0.01% B, 0.005-0.03% Ti, 0.005%-0.06% Nb, 0.005-0.1% S, up to 0.006% calcium, up to 0.03% Te, up to 0.05% Se, up to 0.05% Bi and retained iron and unavoidable impurities is cast to form a semi-finished product which is hot-forged in a conventional manner to produce a forged part. The forged part then undergoes heat treatment, comprising cooling at a rate of cooling Vr of more than 0.5° C./s from a temperature at which the steel is austenitic, to a temperature Tm of between Ms+100° C. and Ms−20° C. The forged part is then maintained for at least two minutes at a temperature between the temperature Tm and a temperature Tf, for which Tf>Tm−100° C. applies. In this way the intention is to obtain a steel component with a substantially bainitic structure, comprising at least 15% lower bainite and preferably at least 20% bainite formed between Tm and Tf.

Practical trials with steel materials of the kind described above have shown that such bainitic steels are unsuitable for components with major changes in cross section due to their tendency to warp and highly fluctuating mechanical characteristics.

Against this background, the problem for the invention was to provide a steel having a high strength, without the need to perform complex heat treatment processes, with a low tendency to warping and which as such is particularly for the production by forging techniques of forged parts with major changes in cross section over their length.

The intention is similarly to indicate a forged part which has an optimum combination of properties without complex heat treatment processes.

Finally, the intention is to propose a method for producing a forged part allowing, with simple means, the creation of forged parts with an optimised combination of properties.

With regard to the steel, the invention has solved the abovementioned problem with the engineering steel indicated in claim 1.

Wirth regard to the forged part, the solution according to the invention to the abovementioned problem consists of producing such a steel component from a steel according to the invention.

Lastly, with regard to the method, the invention has solved the abovementioned problem in that in the production of a forged part the process steps mentioned in claim 13 are carried out.

Advantageous forms of the invention are indicated in the dependent claims and will be explained in detail in the following together with the general inventive idea.

An engineering steel according to the invention has a yield strength of at least 750 MPa and a tensile strength of at least 950 MPa and an at least 80 vol.-% bainitic structure, wherein the remaining 20 vol.-% of the structure can be retained austenite, ferrite, perlite or martensite.

Here, the steel according to the invention is characterised by a high elongation at rupture A of at least 10%, in particular of at least 12%, wherein it has been shown in practice that steels according to the invention routinely achieve an elongation at rupture A of at least 15%.

According to the invention, the engineering steel therefore comprises (in wt.-%) up to 0.25% C, up to 1.5% Si, in particular up to 1% Si or up to 0.45% Si, 0.20-2.00% Mn, up to 4.00% Cr, 0.7-3.0% Mo, 0.004-0.020% N, up to 0.40% S, 0.001-0.035 % Al, 0.0005-0.0025% B, up to 0.015% Nb, up to 0.01% Ti, up to 0.50% V, up to 1.5% Ni, up to 2.0% Cu and retained iron and unavoidable impurities, wherein the Al content % Al, the Nb content % Nb, the Ti content % Ti, the V content % V and the N content % N of the engineering steel in each case meet the following condition:

% Al/27+% Nb/45+% Ti/48+% V/25>% N/3.75

The unavoidable impurities resulting from production include all elements which with regard to their properties of interest here are present in quantities that have no effect on the alloying process and due to the steel-making route or the respective starting material selected (scrap) find their way into the steel. The unavoidable impurities also include, in particular, contents of P of up to 0.0035 wt.-%.

A steel according to the invention and the forged parts produced from this can be characterised by a particularly uniform distribution of properties even if, due to variable component dimensions, during cooling from the forging heat, considered across the forged part volume, localised highly different cooling conditions prevail. This insensitivity to the cooling conditions is achieved in that the engineering steel according to the invention has a homogenous, as far as possible exclusively bainitic structure with low variation in hardness. This homogenous microstructure at the same time has low internal stresses, which has a positive influence on the warping behaviour.

Consequently, steel according to the invention is particularly suited to the production of forged components, in which sections with highly differing volumes and diameters come up against one another. Examples of such forged parts, for the manufacture of which using forging techniques the steel according to the invention is particularly suited, are crankshafts, piston rods and similar, intended in particular, for combustion engines.

Furthermore, parts in the area of the chassis and the wheel suspension with highly different cross sections can be reliably produced from the steel according to the invention without major post-processing through grinding while maintaining the predetermined strength characteristics.

As will be understood from the time-temperature diagram attached as FIG. 1 of a steel according to the invention, from a material technology point of view, this means that with an engineering steel according to the invention a particularly broad window can be used for the bainitic treatment, if the engineering steel according to the invention is continuously cooled from the forging heat. In so doing, the alloying of the engineering steel according to the invention is selected such that in the course of the cooling none of the quantities of martensite or ferrite and/or perlite influencing its properties result in the structure. Engineering steel according to the invention is thus characterised in that it has a predominantly, that is to say up to at least 80 vol.-%, bainitic structure, wherein the content of non-bainitic structural components in steels according to the invention is typically minimised to such an extent that the steel according to the invention has a completely bainitic structure in the technical sense.

Here, with the engineering steel according to the invention an almost constant hardness occurs in the bainite as far as possible independently of the speed of cooling. The constant hardness is a consequence of the almost complete transformation of what was previously austenite into bainite, preferably in a bainitic transformation stage.

Limiting the content of C to a maximum of 0.25 wt.-% means on the one hand that an engineering steel according to the invention despite its maximised strength has good elongation and ductility properties. In a steel according to the invention, the low C content also contributes to accelerating the bainite transformation so that the development of undesired structural components is avoided.

At the same time, however, a certain quantity of carbon in the engineering steel according to the invention can also contribute to the strength. To this end, contents of at least 0.09 wt.-% C in the steel can be envisaged. An optimised effect of the presence of C in the steel according to the invention can thus be achieved in that the C content is adjusted to 0.09-0.25 wt.-%.

The Si content of a steel according to the invention is limited to 1.5 wt.-%, in particular 1 wt.-% or 0.75 wt.-%, to allow the bainite transformation to take place as early as possible. To be particularly sure of achieving this effect, the Si content can also be limited to a maximum of 0.45 wt.-%.

Mo is present in the engineering steel according to the invention in contents of 0.6-3.0 wt.-%, to delay the transformation of the structure into ferrite or perlite. This effect occurs in particular if at least 0.7 wt.-%, in particular more than 0.70 wt.-% Mo, is present in the steel. For contents of more than 3.0 wt.-% no further economically viable increase in the positive effect of Mo occurs in the steel according to the invention. Apart from this, above 3.0 wt.-% Mo there is a danger of formation of a molybdenum-rich carbide phase, which can negatively influence ductility properties. Optimum effects of Mo in the steel according to the invention can be expected if the Mo content is at least 0.7 wt.-%. Here, Mo contents of a maximum of 2.0 wt.% have proven particularly effective.

Manganese is present in contents of 0.20-2.00 wt.-% in the steel according to the invention, in order to adjust the tensile strength and yield strength. A minimum content of 0.20 wt.-% Mn is necessary to achieve an increase in strength. If it is intended to achieve this effect with particular reliability, then an Mn content of at least 0.35 wt.-% may be provided for. Excessively high Mn contents lead to delays in bainite transformation and thus to a predominantly martensitic transformation. The Mn content is therefore limited to a maximum of 2.00 wt.-%, in particular 1.5 wt.-%. Negative influences from the presence of Mn can be particularly reliably avoided by limiting the Mn content in the steel according to the invention to a maximum of 1.1 wt.-%.

The sulphur content of a steel according to the invention can be up to 0.4 wt.-%, in particular max. 0.1 wt.-% or max. 0.05 wt.-%, to support the machinability of the steel.

Fine adjustment of the alloying techniques with regard to the mechanical properties and the microstructure of an engineering steel according to the invention takes place according to the alloying concept according to the invention through combined micro-alloying of the elements of boron in contents of 0.0005-0.0025 wt.-%, nitrogen in contents of 0.004-0.020 wt.-%, in particular at least 0.006 wt.-% N or up to 0.0150 wt.-% N, aluminium in contents of 0.001-0.035 wt.-% and Niob in contents of up to 0.015 wt.-%, titanium in contents of up to 0.01 wt.-% and vanadium in contents of up to 0.10 wt.-%.

Here, the contents of % Al, % Nb, % Ti, % V and % N and Al, Nb, Ti, V and N are linked together by the condition

% Al/27+% Nb/45+% Ti/48+% V/25>% N/3.75

such that the nitrogen contained in the engineering steel through the respective contents present of Al and the necessary contents of Nb, Ti and V also added, is completely bound and boron can thus have a transformation-delaying effect. At the same time, the contents of microelements provided according to the invention and balanced with one another and the N content contribute to an increase in the fine grain stability and strength.

The binding according to the invention of N also allows the boron to be effective as a dissolved element in the matrix and suppresses the formation of ferrite and/or perlite.

To allow advantage to be taken reliably of the microalloying elements and of aluminium, it may be expedient to set the Al content to at least 0.004 wt.-%, the Ti content to at least 0.001 wt.-%, the V content to at least 0.02 wt.-% or the Nb content to at least 0.003 wt.-%. Here, the microalloying elements V, Ti and Nb, on the one hand, and Al, on the other, can in each case be present in combination with one or more elements from the group “Al, V, Ti, Nb” or alone in quantities above the stated minimum contents.

For contents of up to 0.008 wt.-% Ti, of up to 0.01 wt.-% Nb, of up to 0.075 wt.-% V or of up to 0.020 wt.-% Al, the actions of these elements in the construction steel according to the invention can be used to particularly good effect. At the same time, the nitrides or carbonitrides formed lead to an increase in the strength and contribute to the fine grain stability. Here also, the stated upper limits of the contents of Ti, Nb, V or Al in each case alone or in combination with one another can be adhered to in order to achieve the optimum effect of the alloying element concerned.

Optionally present contents of Cr of up to 4.00 wt.-%, in particular up to 3 wt.-% or up to 2.5 wt.-%, contribute to the durability and the corrosion-resistance of the steel according to the invention. To this end, by way of example, at least 0.5 wt.-% or at least 0.8 wt.-% Cr can be provided.

Similarly optionally present contents of Ni of up to 1.5 wt.-% can likewise contribute to the hardenability of the steel.

The alloying elements reaching the steel according to the invention via the starting material or intentionally added alloying elements also include Cu, the content of which, in order to avoid negative influences in the steel according to the invention is limited to a maximum of 2.0 wt.-%. A positive effect of the optional presence of copper in the alloying of an engineering steel according to the invention consists of the formation of the finest retained austenite films and the associated significant raising of the level of ductility. This effect can be achieved in that at least 0.3 wt.-% Cu, in particular more than 0.3 wt.-% Cu, is present in the engineering steel according to the invention. By limiting the Cu content to a maximum of 0.9 wt.-%, an optimised positive effect of the copper content can be achieved.

If a steel according to the invention is heated to heat temperatures typical for hot working of at least 100° C. above the respective Ac3 temperature, in particular a heat temperature of more than 900° C. for the hot working, then hot worked and finally cooled in a regulated or unregulated fashion under stationary or moving air to a temperature of less than 200° C., in particular to room temperature, then over an extremely broad range of cooling speeds following transformation a uniform bainitic structure results. The Ac3 temperature of the steel can be determined in a known manner on the basis of its respective composition. The upper limit to the range of the heating temperature is typically 1,300° C., in particular 1,250° C. or 1,200° C.

As a dimension for the range of cooling speeds, the t8/5 time can be used here, thus the time taken by the respective hot-worked part to cool from 800° C. to 500° C. For the cooling of components produced from the steel according to the invention, this t8/5 time is intended to be 10-1,000 s.

The cooling time selected in each specific case should be selected based on the respective heating temperature. The influence of the heating temperature can be understood from the time-temperature diagram attached as FIG. 2, in which for the heating temperatures 900° C. (unbroken line), 1,100° C. (dashed line) and 1,300° C. (dotted line) the respective position of the respective bainite range is shown across the cooling time. Accordingly, at low heating temperatures of 900° C. shorter t8/5 times should be selected, to achieve the desired bainitic structure, whereas at higher heating temperatures the cooling can be slower. A high certainty that during cooling of steel according to the invention the bainite range will be reached independently of the respective heating temperature exists for steels according to the invention at heating temperatures in the range of 900-1,300° C. and accordingly if the t8/5 time is 100-800 s.

The alloying concept according to the invention therefore allows high hot-working temperatures of more than 1,150° C., as a result of which the forming forces during the hot-working can be reduced without an undesired grain growth occurring.

The method according to the invention for producing forged parts with a yield strength of at least 750 MPa and a tensile strength of at least 950 MPa and an at least 80 vol.-% bainitic structure, which can contain in total up to 20 vol.-% of retained austenite, ferrite, perlite or martensite, comprises the following process steps:

providing a semi-finished product for forging, comprising an engineering steel with a composition according to the invention as explained above;

heating the semi-finished product for forging to a forging temperature of at least 100° C. above the Ac3 temperature of the respective engineering steel, wherein the Ac3 temperature is determined in a conventional manner as a function of the respective composition of the engineering steel;

forging the semi-finished product for forging heated to the forging temperature into the forged part;

cooling the forged part from the forging heat to a temperature of below 500° C., wherein the t8/5-time for cooling is 10-1,000 s

To reduce the forming forces, in the course of the method according to the invention with regard to minimisation of the necessary forging forces, it may prove advantageous if the respective semi-finished product representing the starting point of the forging is heated to a forging temperature of more than 1,150° C.

A further adjustment of the mechanical properties, in particular the strength and ductility of the components hot-worked, in particular forged, from steel according to the invention, can take place by means of tempering treatment, during which the respective part is maintained for a duration of 0.5-2 h in the temperature range 180-375° C.

In practice, with the steel according to the invention, tensile strengths of at least 950 MPa, a yield strength of at least 750 MPa, and an elongation at rupture A of at least 15%, wherein it has been shown in practice that even higher elongation values A of at least 17%, can be reliably achieved. This combination of features in forged parts comprising steel according to the invention in particular if they are created in the manner according to the invention.

In the following the invention is explained in more detail using exemplary embodiments.

Steel melts E1-E6 according to the invention and a comparison melt V1 with the compositions shown in Table 1 were smelted and cast into semi-finished products, which involved blocks, as normally made available for further processing using forging techniques.

The semi-finished products are heated for forging deformation to a heat temperature Tw, then in a conventional manner hot worked using drop forging to produce forged parts and then cooled to room temperature in the air. With some of the forged parts obtained, a tempering treatment is then performed.

Table 2 shows the heating temperatures Tw applied in the examples, the t8/5 time necessary in each case for passing through the critical temperature range of 800-500° C., the temperature and duration of the tempering treatment, where this was actually carried out, and the proportion of bainite in the structure, the tensile strength Rm, the yield strength Re, the extension A and the notch impact energy W of the forged part obtained after forging.

The examples show that when the specifications according to the invention are met, forged parts can be produced which allow the operating parameters set during their creation to be varied over a wide range and in so doing to obtain hot-worked components with optimised mechanical properties.

TABLE 1 Steel C Si Mn Cr Mo N S Al B E1 0.13 0.4 0.55 2.37 1.04 0.0069 0.003 0.015 0.0012 E2 0.17 0.25 0.72 2.05 0.71 0.0100 0.005 0.020 0.0012 E3 0.17 0.24 0.90 1.72 0.74 0.0082 0.003 0.031 0.0008 E4 0.23 0.27 0.43 1.23 0.77 0.0076 0.034 0.017 0.0013 E5 0.16 0.73 1.49 0.94 0.78 0.0077 0.004 0.027 0.0013 E6 0.19 0.67 0.89 1.47 0.79 0.0092 0.005 0.035 0.0012 V1 0.24 0.10 1.50 2.00 0.03 0.0100 0.002 0.023 — Steel Nb Ti V Ni Cu p (1) (2) (1) > (2) E1 0.003 0.002 0.03 0.24 0.19 0.019 0.006864 0.00184 YES E2 0.021 0.001 0.10 0.24 0.23 0.021 0.005228 0.002667 YES E3 0.007 0.001 0.03 0.22 0.62 0.017 0.002552 0.002187 YES E4 0.003 0.001 0.04 0.17 0.21 0.017 0.002317 0.002027 YES E5 0.003 0.001 0.06 0.21 0.17 0.016 0.003488 0.002050 YES E6 0.003 0.001 0.03 0.22 0.13 0.020 0.002584 0.002453 YES V1 0.020 0.015 0.02 0.40 0.50 0.018 0.002409 0.002667 NO Data in wt.-%, retained iron and unavoidable impurities (1): %Al/27 + %Nb/45 + %Ti/48 + %V/25 (2): %N/3,75

TABLE 2 Tempering Proportion of Tw t8/5 treatment bainite in structure Rm Re A According to Steel [° C.] [s] [° C.], [h] [vol.-%] [MPa] [MPa] [%] the invention? E1 1050 320 None >97% 965 763 22 YES E2 1080 580 None >97% 1225 972 17 YES E3 1080 640 None >97% 1174 840 25 YES E4 1150 500 300° C., 1.5h >97% 1192 1034 24 YES E5 950 100 None >97% 1353 1112 24 YES E6 950 200 None >97% 1367 1167 22 YES V1 1075 500 None   75% 1352 897 8 NO (Remainder MS) 

1. An engineering steel having a yield strength of at least 750 MPa, a tensile strength of at least 950 MPa and a structure consisting of at least 80 vol.-% of bainite and in total a maximum of 20 vol.-% of retained austenite, ferrite, perlite and/or martensite, wherein the steel comprises (in wt.-%) C: 0-0.25%, Si: 0-1.5%, Mn: 0.20-2.00%, Cr: 0-4.00%, Mo: 0.6-3.0%, N: 0.004-0.020%, S: 0-0.40%, Al: 0.001-0.035%, B: 0.0005-0.0025%, Nb: 0-0.015%, Ti: 0-0.01%, V: 0-0.10%, Ni: 0-1.5%, Cu: 0-2.0%, remainder iron and unavoidable impurities, and the Al content % Al, the Nb content % Nb, the Ti content % Ti, the V content % V and the N content % N of the engineering steel in each case meet the following condition: % Al/27+% Nb/45+% Ti/48+% V/25>% N/3.75.
 2. The engineering steel according to claim 1, wherein the C content is at least 0.09 wt.-%.
 3. The engineering steel according to claim 1, wherein the Al content is at least 0.004 wt.-%.
 4. The engineering steel according to claim 1, wherein the Al content is a maximum of 0.020 wt.-%.
 5. The engineering steel according to claim 1, wherein the Nb content is at least 0.003 wt.-%.
 6. The engineering steel according to claim 1, wherein the Nb content is a maximum of 0.01 wt.-%.
 7. The engineering steel according to claim 1, wherein the Ti content is at least 0.001 wt.-%.
 8. The engineering steel according to claim 1, wherein the Ti content is a maximum of 0.008 wt.-%.
 9. The engineering steel according to claim 1, wherein the V content is at least 0.02 wt.-%.
 10. The engineering steel according to claim 1, wherein the V content is a maximum of 0.075 wt.-%.
 11. The engineering steel according to claim 1, wherein the elongation at rupture A is at least 10%.
 12. A forged part comprising a steel in accordance with claim
 1. 13. A method for producing a forged part having a yield strength of at least 750 MPa and a tensile strength of at least 950 MPa and an at least 80 vol.-% bainitic structure, wherein the remaining maximum of 20 vol.-% of other proportions of the structure are retained austenite, ferrite, perlite and/or martensite, comprising the following process steps: a. providing a semi-finished product for forging, comprising an engineering steel with a composition according to claim 1; b. heating the semi-finished product for forging to a forging temperature of at least 100° C. above the Ac3 temperature of the engineering steel; c. forging the semi-finished product for forging heated to the forging temperature into the forged part; d. cooling the forged part from the forging temperature to a temperature of below 200° C., wherein the t8/5-time for cooling is 10-1,000 s.
 14. The method according to claim 13, wherein the forging temperature is higher than 1,150° C.
 15. The method according to claim 13, wherein following cooling the forged part undergoes tempering treatment, during which it is maintained for a duration of 0.5-2 hours at a tempering temperature of 180-375° C. 